clang++-19 -std=gnu++14 -Wc++20-extensions wrongly warns about GNU variadic macro extension

[jinkama-san]

Hi,

for simple code like

#ifdef DEBUG
# define DBG(fmt, ...) printf(fmt, ##__VA_ARGS__)
#else
# define DBG(fmt, ...)
#endif
#include <stdio.h>
int main()
{
        DBG("Test\n");
}

compiled with clang++-19 -std=gnu++14 test-variadic-macro.cpp -Wc++20-extensions now this incorrect (for C++14 with GNU extensions) warning shows:

test-variadic-macro.cpp:11:14: warning: passing no argument for the '...' parameter of a variadic macro is a C++20 extension [-Wc++20-extensions]
   11 |         DBG("Test\n");
      |                     ^
test-variadic-macro.cpp:4:10: note: macro 'DBG' defined here
    4 | # define DBG(fmt, ...)
      |          ^
1 warning generated.

This breaks a lot of code in release mode compiles. Also adding -Wno-gnu-zero-variadic-macro-arguments after -Wc++20-extensions doesn't help here. All earlier clang versions I tested including clang-18.1.8 got this right and didn't warn.
Maybe related to issue #76375

[shafik]

Look like @Sirraide added this in 5231005

The warning looks correct but maybe we need a way to suppress it.

CC @erichkeane

[erichkeane]

Also adding -Wno-gnu-zero-variadic-macro-arguments after -Wc++20-extensions doesn't help here.

This is likely the bug here, the warning is correct. Its a GNU extension but it is also a C++20 extension. But the fact that we're not able to disable it with its flag if the higher-level flag is enabled is problematic.

[Sirraide]

Look like @Sirraide added this in 5231005

Iirc the warning was already there before, and I just changed it to allow it for C23 or sth like that, but that was a while ago, so I’m not 100% sure anymore.

[jinkama-san]

Or maybe, regardless of any warning options, suppress the warning always if none of the named arguments are actually used?
For macros there is never (AFAICT) a warning when a named argument is not used, so a warning after an unused named argument doesn't make much sense to me.